This invention relates generally to golf balls and golf ball compositions and, more particularly, to golf balls and golf ball compositions designed to optimize ball performance. This invention also relates to methods of manufacture of such golf balls and golf ball compositions.
Referring to FIGS. 2a-c, golf balls 10-14 generally include a core 16-20 and at least one layer 22-26 surrounding the core. Balls can be classified as two-piece, wound, or multilayer balls 10, 12, and 14, respectively. Two-piece balls 10 include a spherical inner core 16 and an outer cover layer 22. Wound balls 14 include a core 18, a rubber thread (not shown) wound under tension around the core to a desired diameter forming an intermediate layer 28, and a cover layer 24. Cover layers for wound ball generally are made of trans-polyisoprene or thermoset polyurethane. Multilayer balls 14 include a core 20, a cover layer 26, and one or more intermediate layers 30.
Generally, two-piece balls 10 have good durability and distance when hit, but poor “feel”—the overall sensation transmitted to the golfer while hitting the ball—and low spin rate, which results in poor ball control. Wound balls 12 having balata covers 24 generally have high spin rate, leading to good control, and they also have good feel, but they have poor durability and short distance in comparison to two-piece balls. Multi-layer balls 14 generally have performance characteristics between those of two-piece and wound balls; that is, multi-layer balls exhibit durability and distance inferior to two-piece balls but superior to wound balata balls, and they exhibit feel and spin rate inferior to wound balata balls but superior to two-piece balls.
Material characteristics of the compositions used in the core 16-20, cover 22-26, and any intermediate layers 28 and 30 of golf balls 10-14 are among the important factors that determine the performance of the balls. In particular, the composition of the cover layer is important in determining the ball's durability, shear-cut resistance, speed, spin rate, hitting sound (the sound made by a golf club head when it hits the ball), and feel. The composition of an intermediate layer is important in determining the ball's spin rate and speed. Various materials having different physical properties are used to make cover and intermediate layers to create a ball having the most desirable performance possible. In particular, cover layers of many commercially available balls are made using soft or hard ionomeric resins, elastomeric resins, or blends of these.
Ionomeric resins used generally are ionomeric copolymers of an olefin and a metal salt of a unsaturated carboxylic acid, or ionomeric terpolymers having a co-monomer within its structure. These resins vary in resiliency, flexural modulus, and hardness, based on differing acid content, degree of neutralization, and metal cation used for neutralization. Examples of these resins include those marketed under the trademark SURLYN manufactured by E. I. DuPont de Nemours & Company of Wilmington, Del., and IOTEK manufactured by Exxon Mobil Corporation of Irving, Tex. Ionomeric copolymers have been particularly favored for use in golf ball covers 22-26 because they produce ball covers having excellent durability and high resilience. Ionomeric terpolymers are used to produce covers having improved spin and feel, though at the cost of ball speed and durability. Elastomeric resins used in golf ball covers include a variety of thermoplastic or thermoset elastomers available, such as polyurethane, polyetherester elastomer, and polyamide elastomer.
Each of the materials discussed above has particular characteristics that can lead to good golf ball properties when used in a golf ball composition, either for making a ball cover 22-26 or intermediate layer 28 and 30. However, one material generally cannot optimize all of the important properties of a golf ball layer. Properties such as feel, speed, spin rate, resilience, and durability all are important, but improvement of one of these properties by use of a particular material often can lead to worsening of another. For example, ideally, a golf ball cover should have good feel and controllability, without sacrificing ball speed, distance, or durability. Despite the broad use of copolymeric ionomers in golf balls 10-14, their use alone in, for example, a ball cover can be unsatisfactory. A cover providing good durability, controllability, and feel would be difficult to make using only a copolymeric ionomer resin having a high flexural modulus, because the resulting cover, while having good distance and durability, also will have poor feel and low spin rate, leading to reduced controllability of the ball. Also, use of particular elastomeric resins alone can lead to compositions having unsatisfactory properties, such as poor durability and low ball speed.
Therefore, to improve golf ball properties, the materials discussed above can be blended to produce improved ball layers 22-30. Prior compositions for golf balls 10-14 have involved blending high-modulus copolymeric ionomer with, for example, lower-modulus copolymeric ionomer, terpolymeric ionomer, or elastomer. As discussed above, ideally a golf ball cover 22-26 should provide good feel and controllability, without sacrificing the ball's distance and durability. Therefore, a copolymeric ionomer having a high flexural modulus often is combined in a cover composition with a terpolymeric ionomer or an elastomer having a low flexural modulus. The resulting intermediate-modulus blend possesses a good combination of hardness, spin, and durability.
However, even with blending of materials to improve properties, use of the materials and blends discussed above has not been completely satisfactory. Improving one characteristic can lead to worsening another. For example, blending an ionomer having a high flexural modulus with an ionomer having a low flexural modulus can lead to reduced resilience and durability compared to use of the high-modulus ionomer alone. Also, the hardnesses of the compositions that can be obtained from these blends are limited, because durability and resilience get worse when hardness is lowed by increasing terpolymeric content of these blends. In general, it is difficult to make materials for use in, for example, a golf ball cover layer 22-26 that have good feel, high speed, high resilience, and good shear durability, and that are within a wide range of hardness. Additional compositions meeting these criteria therefore are needed.
In view of the above, it is apparent that golf ball compositions are needed that allow the optimization of many ball performance properties without the worsening of other properties. The ball compositions also should provide little or no processing and preparation difficulties over existing compositions. The present invention fulfills this need and other needs, and provides further related advantages.